The fragment molecular-orbital(FMO)method was combined with the single-reference coupled-cluster(CC)theory.The developed method(FMO-CC)was applied at the CCSD and CCSD(T)levels of theory,for the cc-pVnZ family of basis sets(n=D,T,Q)to water clusters and glycine oligomers(up to 32 molecules/residues using as large basis sets as possible for the given system).The two-and three-body FMO-CC results are discussed at length,with emphasis on the basis-set dependence and three-body effects.Two-and three-body approximations based on interfragment distances were developed and the values appropriate for their accurate application carefully determined.The error in recovering the correlation energy was several millihartree for the two-body FMO-CC method and in the submillihartree range for the three-body FMO-CC method.In the largest calculations,we were able to perform the CCSD(T)calculations of(H_2O)_(32)with the cc-pVQZ basis set(3680 basis functions)and(GLY)_(32)with the cc-VDZ basis set(712 correlated electrons).FMO-CC was parallelized using the upper level of the two-layer parallelization scheme.The computational scaling of the two-body FMO-CC method was demonstrated to be nearly linear.As an example of timings,CCSD(T)calculations of(H_2O)32 with cc-pVDZ took 13 min on an eight node 3.2-GHz Pentium4 cluster.
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